Nucleation and growth of thin films of rod--like conjugated molecules

Thin films formed from small molecules rapidly gain importance in different technological fields. To explain their growth, methods developed for zero--dimensional atoms as the film forming particles are applied. However, in organic thin film growth the dimensionality of the building blocks comes into play. Using the special case of the model molecule para--Sexiphenyl, we will emphasize the challenges that arise from the anisotropic and one--dimensional nature of building blocks. Differences or common features with other rodlike molecules will be discussed. The typical morphologies encountered for this group of molecules and the relevant growth modes will be investigated. Special attention is given to the transition between flat lying and upright orientation of the building blocks during nucleation. We will further discuss methods to control the molecular orientation and describe the involved diffusion processes qualitatively and quantitatively.Comment: as submitted to JPCM (revised version) fixed figures and a few lines of tex

Influence of defect-induced deformations on electron transport in carbon nanotubes

We theoretically investigate the influence of defect-induced long-range deformations in carbon nanotubes on their electronic transport properties. To this end we perform numerical ab-initio calculations using a density-functional-based tight-binding (DFTB) model for various tubes with vacancies. The geometry optimization leads to a change of the atomic positions. There is a strong reconstruction of the atoms near the defect (called "distortion") and there is an additional long-range deformation. The impact of both structural features on the conductance is systematically investigated. We compare short and long CNTs of different kinds with and without long-range deformation. We find for the very thin (9,0)-CNT that the long-range deformation additionally affects the transmission spectrum and the conductance compared to the short-range lattice distortion. The conductance of the larger (11,0)- or the (14,0)-CNT is overall less affected implying that the influence of the long-range deformation decreases with increasing tube diameter. Furthermore, the effect can be either positive or negative depending on the CNT type and the defect type. Our results indicate that the long-range deformation must be included in order to reliably describe the electronic structure of defective, small-diameter zigzag tubes.Comment: Materials for Advanced Metallization 201

Arbeitsmarkteinstieg und räumliche Mobilität von Hochschulabsolvent*innen

There is a growing demand for (higher) education and qualified human capital in knowledge-based economies, with human capital being crucial for economic growth and regional development. The scarcity of skilled workers and the simultaneous increase in the requirements of jobs lead to educational expansion. However, the exploitation of human capital is tied to the application of its respective skills and knowledge in the labor market. The societal and individual financial returns to education also only take effect upon successful entry into the labor market. Accordingly, it is the task of research in higher education, among others, to illuminate the transition phase into the labor market from individual, societal and economical perspectives. Against this background, this dissertation has two research goals. Firstly, it aims to address the empirical challenges that previous studies face by developing a novel and unique dataset. Secondly, the dissertation aims to incorporate the hitherto neglected interplay of three factors of labor market entry (work experience, field of study and mobility) into both empirical models and theoretical frameworks. Matching administrative student data from several German universities with employment biographies from social security records at the individual level enabled the creation of a panel data set of graduates. Using this panel, the labor market entry as well as early career paths and their respective success factors are analyzed in depth. Key findings suggest that labor market experiences and mobility patterns have significant effects on labor market entry variables such as the geographical location, the duration of the transition from university to employment, wages and the adequacy of employment. However, strength and direction of these effects depend on the type, specificity, location and timing of work experiences. In this dissertation, a conceptual categorization of the relation between fields of study and their associated labor markets allows the identification of important effects of the field of study on labor market performances. In more detail, the labor market entry of graduates from different fields, i.e. geography, business, computer sciences, is systematically compared, and differences as well as their predictors are quantified. Results suggest that geographers perform worse than their business and computer science peers do. However, this is not a problem of geographers per se but rather due to the indirect and multidimensional links between some fields of study and their respective labor markets. The insights gained through this dissertation help to better understand the mechanisms underlying the labor market entry of graduates, leading to practical implications in several domains such as the planning of educational trajectories by individuals, the improvement of skilled labor recruitment strategies by firms, and the decision-making in regional policy.In Wissensgesellschaften ist der Bedarf an Bildung und qualifiziertem Humankapital hoch, ihre ökonomische Relevanz für Wachstum und Entwicklung sowie die Bedeutung für das Individuum sind unbestritten. Aus dem Fachkräftemangel und der gleichzeitigen Zunahme der Anforderungen der beruflichen Tätigkeiten entsteht eine Bildungsexpansion, um die so entstandene Nachfrage zu bedienen; die Aktivierung der dadurch erhöhten Potenziale des qualifizierten Humankapitals ist dabei jedoch an die Umsetzung seiner spezialisierten Fähigkeiten und Wissen im Arbeitsmarkt gebunden. Dabei greifen die gesellschaftlichen und individuellen, monetären Bildungsrenditen erst bei erfolgreichem Arbeitsmarkteinstieg. Entsprechend ist es Aufgabe unter anderem der Hochschul-absolvent*innenforschung, die komplexen Bedingungen, denen die Eintrittsphase in das Arbeitsleben unterliegt, sowohl aus individueller, gesellschaftlicher als auch arbeitsmarktökonomischer Perspektive zu beleuchten. Diese Dissertation beschäftigt sich entsprechend mit zwei Forschungszielen: einerseits, die empirischen Herausforderungen bisheriger Studien mit einem neuartigen, unikalen Datensatz zu adressieren. Andererseits, drei im Zusammenwirken bis dato wenig beobachtete Faktoren des Arbeitsmarkteinstiegs (Arbeitserfahrung, Mobilitätsverhalten, Studienfach) in die Analysen sowie in theoretische Forschungsansätze zu integrieren. Hierdurch kann ein vertieftes Verständnis der Übergangsphase zwischen Hochschulabschluss und Arbeitsmarkteinstieg gewonnen werden, wie es bisher in diesem Kontext nicht vorliegt. Durch die Verknüpfung von administrativen Daten Studierender mehrerer deutscher Hochschulen mit Erwerbsbiographien der Sozialversicherungen auf Individualebene ist es gelungen, ein Hochschul-absolvent*innenpanel zu erstellen, mit dessen Hilfe die Erfolgsfaktoren des Arbeitsmarkteinstiegs und des frühen Karriereverlaufs detailliert und differenziert analysiert werden. Zentrale Ergebnisse zeigen, dass bereits vorhandene Erfahrungen auf dem Arbeitsmarkt und Mobilitätsverhalten bedeutsame Wirkung auf Parameter des Arbeitsmarkteinstieges – wie räumliche Verortung, Länge der Übergangsphase zwischen Hochschule und Arbeitsmarkt, Entgelt, Adäquanz der Beschäftigung etc. – haben. Die Stärke und die Effektrichtung hängen allerdings deutlich von der Art, Spezifität, Ort und Zeit der Arbeitserfahrungen ab. Die konzeptionelle Kategorisierung der Beziehungen von Studienfach und assoziiertem Arbeitsmarkt ermöglicht, die signifikanten Effekte, die das Studienfach auf die Arbeitsmarktperformance hat, zu erkennen. Im hier erstmals vorgenommenen systematischen Vergleich des Arbeitsmarkteinstiegs von Absolvent*innen der Geographie, Wirtschaftswissenschaften und Informatik gelingt es, die Unterschiede genau zu quantifizieren und Wirkungsfaktoren zu identifizieren. Hierbei wird deutlich, dass die Herausforderungen, denen Geograph*innen im Arbeitsmarkt gegenüberstehen, kein Alleinstellungsmerkmal des Faches sind, sondern struktureller Natur bei Fächern mit indirekten, multidimensionalen Verbindungen zu ihrem jeweiligen Arbeitsmarkt. Die durch diese Dissertation gewonnenen Erkenntnisse helfen, die Ausgestaltung und die Mechanismen hinter dem Arbeitsmarkteinstieg von Hochschulabsolvent*innen besser zu verstehen. Sie bietet damit Erkenntnisse für praktische Implikationen bei der Gestaltung von individuellen Bildungsinvestitionen, der Fachkräfterekrutierung und bei hochschulpolitischen Entscheidungen sowohl für (potenzielle) Studierende als auch Akteur*innen aus Hochschule, Wirtschaft und Regionalpolitik

Niche hypergraphs

If D = (V,A) is a digraph, its niche hypergraph Nℋ(D) = (V, ℰ) has the edge set ℰ = {ℯ ⊆ V | |e| ≥ 2 ∧ ∃ v ∈ V : e = ND-(v) ∨ ℯ = ND+(v)}. Niche hypergraphs generalize the well-known niche graphs (see [11]) and are closely related to competition hypergraphs (see [40]) as well as double competition hypergraphs (see [33]). We present several properties of niche hypergraphs of acyclic digraphs

Smooth Growth of Organic Semiconductor Films on Graphene for High-Efficiency Electronics

High-quality thin films of conjugated molecules with smooth interfaces are important to assist the advent of organic electronics. Here, we report on the layer-by-layer growth of the organic semiconductor molecule p-sexiphenyl (6P) on the transparent electrode material graphene. Low energy electron microscopy and micro low energy electron diffraction reveal the morphological and structural evolution of the thin film. The layer-by-layer growth of 6P on graphene proceeds by subsequent adding of {(111)} layers

The influence of substrate temperature on growth of para-sexiphenyl thin films on Ir{111} supported graphene studied by LEEM

The growth of para-sexiphenyl (6P) thin films as a function of substrate temperature on Ir{111} supported graphene flakes has been studied in real-time with Low Energy Electron Microscopy (LEEM). Micro Low Energy Electron Diffraction (\mu LEED) has been used to determine the structure of the different 6P features formed on the surface. We observe the nucleation and growth of a wetting layer consisting of lying molecules in the initial stages of growth. Graphene defects -- wrinkles -- are found to be preferential sites for the nucleation of the wetting layer and of the 6P needles that grow on top of the wetting layer in the later stages of deposition. The molecular structure of the wetting layer and needles is found to be similar. As a result, only a limited number of growth directions are observed for the needles. In contrast, on the bare Ir{111} surface 6P molecules assume an upright orientation. The formation of ramified islands is observed on the bare Ir{111} surface at 320 K and 352 K, whereas at 405 K the formation of a continuous layer of upright standing molecules growing in a step flow like manner is observed.Comment: 9 pages, 7 figures, Revised Version as accepted for publication in Surface Scienc

Design of Friction, Morphology, Wetting, and Protein Affinity by Cellulose Blend Thin Film Composition

Cellulose derivate phase separation in thin films was applied to generate patterned films with distinct surface morphology. Patterned polymer thin films are utilized in electronics, optics, and biotechnology but films based on bio-polymers are scarce. Film formation, roughness, wetting, and patterning are often investigated when it comes to characterization of the films. Frictional properties, on the other hand, have not been studied extensively. We extend the fundamental understanding of spin coated complex cellulose blend films via revealing their surface friction using Friction Force Microscopy (FFM). Two cellulose derivatives were transformed into two-phase blend films with one phase comprising trimethyl silyl cellulose (TMSC) regenerated to cellulose with hydroxyl groups exposed to the film surface. Adjusting the volume fraction of the spin coating solution resulted in variation of the surface fraction with the other, hydroxypropylcellulose stearate (HPCE) phase. The film morphology confirmed lateral and vertical separation and was translated into effective surface fraction. Phase separation as well as regeneration contributed to the surface morphology resulting in roughness variation of the blend films from 1.1 to 19.8 nm depending on the film composition. Friction analysis was successfully established, and then revealed that the friction coefficient of the films could be tuned and the blend films exhibited lowered friction force coefficient compared to the single-component films. Protein affinity of the films was investigated with bovine serum albumin (BSA) and depended mainly on the surface free energy (SFE) while no direct correlation with roughness or friction was found. BSA adsorption on film formed with 1:1 spinning solution volume ratio was an outlier and exhibited unexpected minimum in adsorption

Hierarchy of adhesion forces in patterns of photoreactive surface layers

Precise control of surface properties including electrical characteristics, wettability, and friction is a prerequisite for manufacturing modern organic electronic devices. The successful combination of bottom up approaches for aligning and orienting the molecules and top down techniques to structure the substrate on the nano and micrometer scale allows the cost efficient fabrication and integration of future organic light emitting diodes and organic thin film transistors. One possibility for the top down patterning of a surface is to utilize different surface free energies or wetting properties of a functional group. Here, we used friction force microscopy (FFM) to reveal chemical patterns inscribed by a photolithographic process into a photosensitive surface layer. FFM allowed the simultaneous visualization of at least three different chemical surface terminations. The underlying mechanism is related to changes in the chemical interaction between probe and film surface.Comment: 11 pages, 5 figures The following article has been submitted to Journal of Chemical Physics. After it is published, it will be found at http://jcp.aip.org